The eddy current method of non-destructively evaluating metal products is widely used. Basically the method consists in moving a coil over the item to be tested. A high frequency alternating current in the coil produces an alternating magnetic field. When the magnetic field of the coil intersects the item, eddy currents are induced in the specimen close to its surface. These eddy currents in turn induce a magnetic field in opposition to the primary field around the coil, causing a partial reduction in the field of the coil. This decrease in magnetic flux through the coil causes a change in the impedance of the coil. The impedance caused by the eddy currents is in turn dependent on the resistance these currents encounter as they circulate through the item to be tested. Since flaws on the surface (such as cracks, pits, or regions of local thinning) create regions of higher resistance at the flaw locations, eddy current probes may be used to locate flaws. Eddy current testing is essentially the measurement of changes in the impedance of a probe.
In an AC bridge circuit (commonly used in eddy current testing), the change of impedance in the coil will be reflected by a change in the voltage and phase across the circuit. These changes can be analyzed and displayed with the proper equipment so that flaws can be generally characterized. To be analyzed, the AC signal is usually demodulated in its resistive and reactive components (often referred to as X and Y or real and imaginary components). The components will have similar shapes, but different amplitude depending on the AC signal phase caused by the flaw. The demodulated signals show voltage amplitude variation in time and thus allow physical localization of the flaw on the tested part.
The resistive and reactive components can be subsequently added in a vector sum. The sum is then displayed on an X-Y plane called a phasor diagram. The phasor diagram shows amplitude and phase change of the AC signal over a fixed period of time. The shape produced by flaws on the phasor diagram allows further characterization of flaws as being cracks, scorch marks, rust patches, etc.
Eddy current testing is used in many fields, such as pipe or tube inspection used in the heat exchangers of nuclear steam generators. Lately, eddy current testing has also been used for the inspection of control rods used in the core of the reactor. Usually the control rods are filled with a material which absorbs neutrons readily while the outer shell is made out of a metallic alloy such as INCONEL.TM.. Control rods are set between fuel rods to regulate the rate of nuclear reaction. Withdrawal of the rods permits free passage of neutrons from one fuel rod to another, thus increasing the reaction rate. The control rods are guided between the fuel rods by perforated plates. Since both the control rods and the fuel rods are submersed in water--where small, constant vibrations are present--the control rods have a tendency to rub against the rim of the guiding holes in the plate thus causing damage to their surface.
In eddy current inspections, probes of the prior art generally come in one of the two following configurations: the encircling probe and the rotating probe. An encircling probe according to the prior art can be characterized by the arrangement shown in FIG. 1A which produces an output signal illustrated in FIG. 1B as the coil 20 moves over the flaw 24.
In an encircling probe (FIG. 1A), a circular coil 20 encircles the cylindrical item 22 to be inspected and moves along its length. When a flaw 24 (such as a lengthwise crack or a rusted area) is encountered, the probe registers a change in voltage across the testing circuit. The demodulated signals will simply show a voltage change over a certain period of time (FIG. 1B). The general localization and overall importance of the flaw can then be deduced.
Though mechanically simple to implement, this configuration does not allow exact flaw size measurement and localization. The response from such a probe does not permit differentiation between, for instance, four small holes and a single large one. The information obtained is the lengthwise position along the rod (or tube) where the flaw is present and relative size of the flaw. In no way can its angular position and exact size be characterized.
A rotating probe according to the prior art is generally represented in FIG. 2A which produces an output signal illustrated in FIG. 2B. In the case of rotating probes (FIG. 2A), a small energized coil 26 orbits around the cylindrical item 22 to be inspected, while traveling along its length. This results in a helical path. Since the path of the coil 26 takes it over the flaw 24 at a certain angle relative to that flaw, the probe can record its width. While the coil orbits around the tube, it passes a certain number of times over the flaw. The signal given by the probe (in terms of resistive and reactive components) resembles a series of "humps" 25 (FIG. 2B) occurring over a certain period of time. Each hump 25 is equivalent to the width of the crack surveyed by the coil. The diameter of the tube and the traveling speed of the coil being known variables, the length of the flaw and its position on the tube or rod can then be precisely determined. This kind of probe can thus determine the size, exact location and importance of the flaw.
U.S. Pat. No. 4,855,677 to Clark, Jr. et al shows a probe based on this principle but applied for use inside a tube. In this case, the rotating coil travels over the inner surface of the tube.
However, eddy current probes are often used in hostile environments (underwater, in irradiated areas of nuclear power generators). In the case of external inspections of rods and tubes, rotating probes require a complex mechanical setup for the coil to be able to orbit the rod (or tube) while still being supplied with a high frequency AC signal. Since it is highly impractical to have the AC signal source turning with the probe, slip rings are needed to feed the rotating probe with the AC signal. The friction generated by these sliding contacts creates undesired noise which affects the test results. They are also sensitive to rust and to accumulation of dirt. Probes based on that principle are subject to frequent malfunctions and early wear. This constitutes the major drawback of rotating probes.
Also, with rotating probes, it is possible that between two turns around the rod, the coil might miss a small flaw, depending on the pitch of the path of the coil around the rod. Rotating probes also have another problem known as lift-off. During inspection, it is possible that the probe wobbles, creating a small gap between the coil an the inspected surface. This gap usually affects the accuracy of the test.
A number of patented inventions have been proposed to remedy these various problems. These prior art inventions are generally represented by the arrangement shown in FIG. 3A whose output signal is illustrated in FIG. 3B. They are based on the following principle: an encircling detector coil 28 (FIG. 3A) is mounted at axially displaced locations along the cylindrical part 22 to be inspected. To enhance the response of this detector coil, a field altering object 30 having a high magnetic permeability is mounted in close proximity to the coil. In this manner, the field altering object disrupts the coil magnetic field in continuously varying locations along the part 22 and near the coil 28 itself.
When this assembly is passed over a tube and encounters a flaw, two things will happen. When the coil 28 reaches the flaw, a first change in voltage is recorded by the analyzing circuitry. But when the field altering object 30 orbiting the coil 28 also passes over the flaw 24, an additional change of voltage is recorded. This change is a function of the volume and width of the flaw. This results in the resistive and reactive components having the appearance of two overlapping signals (FIG. 3B): one signal 27 from an encircling probe and one signal 29 from a rotating probe. One is proportional to the length of the flaw, the other characterizes its width, and both characterize its volume.
U.S. Pat. No. 4,203,069 to Davis discloses such a probe for inspecting the interior of tubes that uses this principle. The apparatus comprises an exciter/detector coil and a ferrite element mounted on the perimeter of a barrel, the barrel rotating inside the coil. The coil is energized with a high frequency signal that induces eddy currents in the tube. While this apparatus travels inside the tube, the rotating ferrite disrupts the field generated by the coil. When the probe passes over a flaw, the response is in the form of the desired two overlapping signals. The patent however does not disclose any practical method for inspecting the exterior of rods or tubes.
Another device based on the use of a coil/field altering object combination is disclosed in U.S. Pat. No. 4,673,879 to Harris et al wherein a cylindrical metallic sleeve is rotatably supported about a workpiece path of travel. Two differentially wound energization coils surround the sleeve near two apertures in the sleeve. The coils are energized with a high frequency signal that induces eddy currents in the workpiece. The apertures periodically disrupt the eddy current inducing magnetic fields and enhance signals from the coils indicative of the presence of flaws in the workpiece.
U.S. Pat. No. 4,683,430 to Harris et al also proposes a combination of an encircling coil with a field altering object. In this case two encircling coils are used, one of them comprising a tubular pathway. The pathway is positioned between the coil and the rod. A steel ball rotates inside the pathway, acting as the field altering object. However, use of a steel ball as field altering element is not very practical. Steel being highly electrically conductive by its nature, the ball will also be subject to eddy currents. These eddy currents will in turn affect the response of the probe, making it much harder to analyze.
Tests conducted by the inventor has shown that these types of combination--that is a single coil matched with a single field altering object--result in a signal that is difficult to analyze. The reason being that the strength of the signal generated by the field altering object is weak relatively to the overall signal of the encircling coil. Generally speaking, the portion of signal generated by the field altering object represents around 20% of the overall signal or even less. This makes it very difficult to determine, at the signal analysis stage, what part of the signal is generated by the field altering object. As an example, in the case of a small flaw, the signals generated by the field altering object and the encircling probe would be almost indistinguishable.
Another approach is presented in U.S. Pat. No. 3,694,740 to Bergstrand where two sets of detecting elements are used in conjunction. The first set consists in two coils that are bridge coupled or differentially coupled. A difference in potential across the circuit indicates the presence of a flaw. However, when the two coils are simultaneously placed over a long flaw, the circuit will respond as if no flaw was present. To compensate for this, a second set consisting of a pair of Hall effect elements--connected to a differential amplifier--orbits the inspected part near the coils. A positive or negative output of the amplifier indicates the presence of a flaw. This type of probe possesses two major drawbacks. First, if the flaw is two dimensional (which would be the case of a long patch of rust), there is a strong possibility that both Hall effect elements would be over the flaw simultaneously. The differential amplifier would then also respond as if there were no flaw. As a result, the rust patch would go undetected. The second drawback resides in the necessity of slip rings and brushes to feed power and remove the signal from the rotating elements. This makes this type of probe just as prone to early wear and breakage as the rotating probe described above and disclosed by Clark.